Change due date in ISU Billing document

We have generated billing and invoicing for number of customers. We want to extend due date i.e. change due date for the billing document so that interest or LPC not charged in next billing and invoicing.

When we go to FPE2M and enter due date and save it gives error that:

Documents with origins R4 and R5 do not allow changes to the due date

Message no. EK329

Diagnosis

The billing has been completed for these documents and they have been printed out.

Is there any other way by which we can change due date for invoicing document.

Regards,

Narendra